Magnetic field imaging with microfabricated optically-pumped magnetometers.

A multichannel imaging system is presented, consisting of 25 microfabricated optically-pumped magnetometers. The sensor probes have a footprint of less than 1 cm2 and a sensitive volume of 1.5 mm × 1.5 mm × 1.5 mm and connect to a control unit through optical fibers of length 5 m. Operating at very low ambient magnetic fields, the sensor array has an average magnetic sensitivity of 24 fT/Hz1/2, with a standard deviation of 5 fT/Hz1/2 when the noise of each sensor is averaged between 10 and 50 Hz. Operating in Earth's magnetic field, the magnetometers have a field sensitivity around 5 pT/Hz1/2. The vacuum-packaged sensor heads are optically heated and consume on average 76 ± 7 mW of power each. The heating power is provided by an array of eight diode lasers. Magnetic field imaging of small probe coils was obtained with the sensor array and fits to the expected field pattern agree well with the measured data.

Efficient drug-delivery using magnetic nanoparticles--biodistribution and therapeutic effects in tumour bearing rabbits.

To treat tumours efficiently and spare normal tissues, targeted drug delivery is a promising alternative to conventional, systemic administered chemotherapy. Drug-carrying magnetic nanoparticles can be concentrated in tumours by external magnetic fields, preventing the nanomaterial from being cleared by metabolic burden before reaching the tumour. Therefore in Magnetic Drug Targeting (MDT) the favoured mode of application is believed to be intra-arterial. Here, we show that a simple yet versatile magnetic carrier-system (hydrodynamic particles diameter <200nm) accumulates the chemotherapeutic drug mitoxantrone efficiently in tumours. With MDT we observed the following drug accumulations relative to the recovery from all investigated tissues: tumour region: 57.2%, liver: 14.4%, kidneys: 15.2%. Systemic intra-venous application revealed different results: tumour region: 0.7%, liver: 14.4 % and kidneys: 77.8%. The therapeutic outcome was demonstrated by complete tumour remissions and a survival probability of 26.7% (P=0.0075). These results are confirming former pilot experiments and implying a milestone towards clinical studies. FROM THE CLINICAL EDITOR: This team of investigators studied drug carrying nanoparticles for magnetic drug targeting (MDT), demonstrating the importance of intra-arterial administration resulting in improved clinical outcomes in the studied animal model compared with intra-venous.

Magnetorelaxometry assisting biomedical applications of magnetic nanoparticles.

Due to their biocompatibility and small size, iron oxide magnetic nanoparticles (MNP) can be guided to virtually every biological environment. MNP are susceptible to external magnetic fields and can thus be used for transport of drugs and genes, for heat generation in magnetic hyperthermia or for contrast enhancement in magnetic resonance imaging of biological tissue. At the same time, their magnetic properties allow one to develop sensitive and specific measurement methods to non-invasively detect MNP, to quantify MNP distribution in tissue and to determine their binding state. In this article, we review the application of magnetorelaxometry (MRX) for MNP detection. The underlying physical properties of MNP responsible for the generation of the MRX signal with its characteristic parameters of relaxation amplitude and relaxation time are described. Existing single and multi-channel MRX devices are reviewed. Finally, we thoroughly describe some applications of MRX to cellular MNP quantification, MNP organ distribution and MNP-based binding assays. Providing specific MNP signals, a detection limit down to a few nanogram MNP, in-vivo capability in conscious animals and measurement times of a few seconds, MRX is a valuable tool to improve the application of MNP for diagnostic and therapeutic purposes.

Magnetized aerosols comprising superparamagnetic iron oxide nanoparticles improve targeted drug and gene delivery to the lung.

PURPOSE: Targeted delivery of aerosols could not only improve efficacy of inhaled drugs but also reduce side effects resulting from their accumulation in healthy tissue. Here we investigated the impact of magnetized aerosols on model drug accumulation and transgene expression in magnetically targeted lung regions of unanesthetized mice. METHODS: Solutions containing superparamagnetic iron oxide nanoparticles (SPIONs) and model drugs (fluorescein or complexed plasmid DNA) were nebulized to unanesthetized mice under the influence of an external magnetic gradient directed to the lungs. Drug accumulation and transgene expression was subsequently measured at different time points. RESULTS: We could demonstrate 2-3 fold higher accumulation of the model drug fluorescein and specific transgene expression in lung regions of mice which had been exposed to an external magnetic gradient during nebulization compared to the control mice without any exposure to magnetic gradient. CONCLUSIONS: Magnetized aerosols present themselves as an efficient approach for targeted pulmonary delivery of drugs and gene therapeutic agents in order to treat localized diseases of the deeper airways.

Identification of magnetic nanoparticles for combined positioning and lentiviral transduction of endothelial cells.

PURPOSE: The combination of magnetic nanoparticles (MNPs) with a magnetic field is a powerful approach to enable cell positioning and/or local gene therapy. Because physical requirements for MNPs differ between these two applications we have explored whether the use of different MNPs can provide site-specific positioning combined with efficient viral transduction of endothelial cells (ECs). METHODS: A variety of MNPs was screened for magnetic cell labeling and lentivirus binding. Then two different MNPs were chosen and their combined application was evaluated regarding EC magnetization and transduction efficiency. RESULTS: The combined use of PEI-Mag2 and NDT-Mag1 particles provided both efficient lentiviral transduction and high magnetic responsiveness of ECs that could be even retained within the vascular wall under flow conditions. The use of these MNPs did not affect biological characteristics of ECs like surface marker expression and vascular network formation. Importantly, with this method we could achieve an efficient functional overexpression of endothelial nitric oxide synthase in ECs. CONCLUSIONS: The application of two different MNPs provides optimal results for magnetic labeling of ECs in combination with viral transduction. This novel approach could be very useful for targeted gene therapy ex vivo and site-specific cell replacement in the vascular system.

Combined targeting of lentiviral vectors and positioning of transduced cells by magnetic nanoparticles.

Targeting of viral vectors is a major challenge for in vivo gene delivery, especially after intravascular application. In addition, targeting of the endothelium itself would be of importance for gene-based therapies of vascular disease. Here, we used magnetic nanoparticles (MNPs) to combine cell transduction and positioning in the vascular system under clinically relevant, nonpermissive conditions, including hydrodynamic forces and hypothermia. The use of MNPs enhanced transduction efficiency of endothelial cells and enabled direct endothelial targeting of lentiviral vectors (LVs) by magnetic force, even in perfused vessels. In addition, application of external magnetic fields to mice significantly changed LV/MNP biodistribution in vivo. LV/MNP-transduced cells exhibited superparamagnetic behavior as measured by magnetorelaxometry, and they were efficiently retained by magnetic fields. The magnetic interactions were strong enough to position MNP-containing endothelial cells at the intima of vessels under physiological flow conditions. Importantly, magnetic positioning of MNP-labeled cells was also achieved in vivo in an injury model of the mouse carotid artery. Intravascular gene targeting can be combined with positioning of the transduced cells via nanomagnetic particles, thereby combining gene- and cell-based therapies.

Minimal-invasive magnetic heating of tumors does not alter intra-tumoral nanoparticle accumulation, allowing for repeated therapy sessions: an in vivo study in mice.

Localized magnetic heating treatments (hyperthermia, thermal ablation) using superparamagnetic iron oxide nanoparticles (MNPs) continue to be an active area of cancer research. For generating the appropriate heat to sufficiently target cell destruction, adequate MNP concentrations need to be accumulated into tumors. Furthermore, the knowledge of MNP bio-distribution after application and additionally after heating is significant, firstly because of the possibility of repeated heating treatments if MNPs remain at the target region and secondly to study potential adverse effects dealing with MNP dilution from the target region over time. In this context, little is known about the behavior of MNPs after intra-tumoral application and magnetic heating. Therefore, the present in vivo study on the bio-distribution of intra-tumorally injected MNPs in mice focused on MNP long term monitoring of pre and post therapy over seven days using multi-channel magnetorelaxometry (MRX). Subsequently, single-channel MRX was adopted to study the bio-distribution of MNPs in internal organs and tumors of sacrificed animals. We found no distinct change of total MNP amounts in vivo during long term monitoring. Most of the MNP amounts remained in the tumors; only a few MNPs were detected in liver and spleen and less than 1% of totally injected MNPs were excreted. Apparently, the application of magnetic heating and the induction of apoptosis did not affect MNP accumulation. Our results indicate that MNP mainly remained within the injection side after magnetic heating over a seven-days-observation and therefore not affecting healthy tissue. As a consequence, localized magnetic heating therapy of tumors might be applied periodically for a better therapeutic outcome.

Nuclear magnetic relaxation in water revisited.

In this study, we revisited nuclear magnetic relaxation of (1)H in water at very low Larmor frequencies that has been studied intensively in earlier years. We make use of the recently developed superconducting quantum interference device based ultra-low field NMR technique, which enables much easier access to the longitudinal spin-lattice relaxation time T(1) and the transversal spin-spin relaxation time T(2) below several kHz than traditional field cycling methods. Our data reproduce and complement the earlier results, in that they corroborate the finding of an exchange process with a correlation time of about 0.34 ms at room temperature which can be attributed to the migration of hydronium and hydroxyl ions in neutral water via hydrogen bridges. The corresponding relaxation process is driven by the interaction of the protons with (17)O and contributes to the T(1) and the T(2) relaxation rate by about 0.12 s(-1). In addition, we found evidence of a very slow exchange process at about 100 Hz that has hitherto not been reported.

A built-in coil system attached to the inside walls of a magnetically shielded room for generating an ultra-high magnetic field homogeneity.

The homogeneity of the magnetic field generated by a coil inside a magnetic shield is essential for many applications, such as ultra-low field nuclear magnetic resonance or spin precession experiments. In the course of upgrading the Berlin Magnetically Shielded Room (BMSR-2) with a new inserted Permalloy layer of side length 2.87 m, we designed a built-in coil consisting of four identical square windings attached to its inside walls. The spacings of the four windings were optimized using a recently developed semi-analytic model and finite element analysis. The result reveals a strong dependence of the field homogeneity on the asymmetric placement of the inner two windings and on the chosen material permeability value µs. However, our model calculations also show that these experimental variations can be counterbalanced by an adjustment of the inner winding positions in the millimeter range. Superconducting quantum interference device-based measurements yield for our implementation after fine adjustments of a single winding position a maximum field change of less than 10 pT for a total field of B0 = 2.3 µT within a 10 cm region along the coil axis, which is already better than the residual field of the upgraded BMSR-2.1 after degaussing. Measurements of free spin precession decay signals of polarized Xe129 nuclei show that the transverse relaxation time for the used cell is not limited by the inhomogeneity of the new built-in coil system.

Differential infraslow (<0.1 Hz) cortical activations in the affected and unaffected hemispheres from patients with subacute stroke demonstrated by noninvasive DC-magnetoencephalography.

BACKGROUND AND PURPOSE: Sustained mass depolarization of neurons, termed cortical spreading depolarization, is one electrophysiological correlate of the ischemic injury of neurons. Cortical spreading depolarizations spread in the gray matter at a rate of approximately 3 mm/min and are associated with large infraslow extracellular potential changes (<0.05 Hz). Moreover, smaller infraslow potential changes accompany functional activation and might help to assess neuronal repair after stroke. The objective of the present pilot study was to investigate whether it is feasible to apply noninvasive near-DC-magnetoencephalography to detect and monitor infraslow field changes in patients with acute stroke. METHODS: A simple motor condition was used to induce physiological cortical infraslow field changes. Five patients in a subacute state after ischemic stroke performed self-paced simple finger movements (30-second periods of finger movements, always separated by 30-second periods of rest, for a total of 15 minutes). Near-DC-magnetoencephalography signals were recorded over the contralateral primary motor cortex for the affected and unaffected hemisphere, respectively. RESULTS: In all patients, the time courses of the contralateral cortical field amplitudes in the infraslow frequency range followed closely the motor task cycles revealing statistically significant differences between finger movement and rest periods. In 4 of 5 patients, infraslow field amplitudes were significantly stronger over the unaffected hemisphere compared with the affected hemisphere. CONCLUSIONS: This study demonstrates that cortical infraslow activity can be recorded noninvasively in patients in the subacute state after ischemic stroke. It is suggested that near-DC-magnetoencephalography is a promising tool to also detect cortical spreading depolarization noninvasively.

Noninvasive monitoring of blood flow using a single magnetic microsphere.

Noninvasive medical imaging of blood flow relies on mapping the transit of a contrast medium bolus injected intravenously. This has the draw-back that the front of the bolus widens until the tissue of interest is reached and quantitative flow parameters are not easy to obtain. Here, we introduce high resolution (millimeter/millisecond) 3D magnetic tracking of a single microsphere locally probing the flow while passing through a vessel. With this, we successfully localize and evaluate diameter constrictions in an arteria phantom after a single passage of a microsphere. We further demonstrate the potential for clinical application by tracking a microsphere smaller than a red blood cell.

Quantification of specific bindings of biomolecules by magnetorelaxometry.

The binding reaction of the biomolecules streptavidin and anti-biotin antibody, both labelled by magnetic nanoparticles (MNP), to biotin coated on agarose beads, was quantified by magnetorelaxometry (MRX). Highly sensitive SQUID-based MRX revealed the immobilization of the MNP caused by the biotin-streptavidin coupling. We found that about 85% of streptavidin-functionalised MNP bound specifically to biotin-agarose beads. On the other hand only 20% of antibiotin-antibody functionalised MNP were specifically bound. Variation of the suspension medium revealed in comparison to phosphate buffer with 0.1% bovine serum albumin a slight change of the binding behaviour in human serum, probably due to the presence of functioning (non heated) serum proteins. Furthermore, in human serum an additional non-specific binding occurs, being independent from the serum protein functionality.The presented homogeneous bead based assay is applicable in simple, uncoated vials and it enables the assessment of the binding kinetics in a volume without liquid flow. The estimated association rate constant for the MNP-labelled streptavidin is by about two orders of magnitude smaller than the value reported for free streptavidin. This is probably due to the relatively large size of the magnetic markers which reduces the diffusion of streptavidin. Furthermore, long time non-exponential kinetics were observed and interpreted as agglutination of the agarose beads.

Improved sensitivity and limit-of-detection using a receive-only coil in magnetic particle imaging.

Magnetic particle imaging (MPI) is an imaging modality capable of quantitatively determining the 3D distribution of a magnetic nanoparticle (MNP) ensemble. In this work, we present a method for reducing the MNP limit of detection by employing a new receive-only coil (Rx-coil) for signal acquisition. The new signal detector is designed to improve the sensitivity and thus quality of reconstructed images. We present characterization measurements conducted with the prototype Rx-coil installed in a preclinical MPI scanner. The gradiometric design of the Rx-coil attenuates the unwanted signal contributions arising from the excitation field, leading to a 17 dB lower background level compared to the conventional dual-purpose coil (TxRx-coil), which is crucial for detecting low amounts of MNP. Network analyzer measurements of the frequency-dependent coil sensitivity, as well as spectral analysis of recorded MPI data demonstrate an overall increase of the coil sensitivity of about +12 dB for the Rx-coil. Comparisons of the sensitivity distributions revealed no significant degradations in terms of homogeneity for the Rx-coil compared to the TxRx-coil in an imaging volume of 6 × 3 × 3 cm3. Finally, the limit of detection was determined experimentally for each coil type using a serial dilution of MNPs, resulting in values of 133 ng of iron for the conventional TxRx-coil and 20 ng for the new Rx-coil, using an acquisition time of 2 s. A linear relationship between the reconstructed signal intensities and the iron mass in the samples was observed with coefficients of determination (R2) of above 99% in the range of the limit of detection to 3 103ng(Fe). These results open the way for improved image quality and faster acquisition time in pre-clinical MPI scanners.

Novel magnetic multicore nanoparticles designed for MPI and other biomedical applications: From synthesis to first in vivo studies.

Synthesis of novel magnetic multicore particles (MCP) in the nano range, involves alkaline precipitation of iron(II) chloride in the presence of atmospheric oxygen. This step yields green rust, which is oxidized to obtain magnetic nanoparticles, which probably consist of a magnetite/maghemite mixed-phase. Final growth and annealing at 90°C in the presence of a large excess of carboxymethyl dextran gives MCP very promising magnetic properties for magnetic particle imaging (MPI), an emerging medical imaging modality, and magnetic resonance imaging (MRI). The magnetic nanoparticles are biocompatible and thus potential candidates for future biomedical applications such as cardiovascular imaging, sentinel lymph node mapping in cancer patients, and stem cell tracking. The new MCP that we introduce here have three times higher magnetic particle spectroscopy performance at lower and middle harmonics and five times higher MPS signal strength at higher harmonics compared with Resovist®. In addition, the new MCP have also an improved in vivo MPI performance compared to Resovist®, and we here report the first in vivo MPI investigation of this new generation of magnetic nanoparticles.

Combined MEG and EEG methodology for non-invasive recording of infraslow activity in the human cortex.

OBJECTIVE: Periinfarct depolarisation and spreading depression represent key mechanisms of neuronal injury after stroke. Changes in cortical electrical potentials and magnetic fields in the very low frequency range are relevant parameters to characterize these events, which up to now have only been recorded invasively. In this study, we proved whether a non-invasive combined MEG/EEG recording technique is able to quantitatively monitor cortical infraslow activity in humans. METHODS: We used repetitive very slow and slow right finger movements as a physiological motor activation paradigm to induce cortical infraslow activity. Infraslow fields were recorded over the left hemisphere using a modulation-based MEG technique. EEG was performed using 16 standard Ag-Cl electrodes that covered the left motor cortex. RESULTS: We recorded stable focal motor-related infraslow magnetic field changes in seven out of seven subjects. We also found correlating infraslow electrical potential changes in three out of seven subjects. Slow finger movements generated significantly stronger field and potential changes than very slow movements. CONCLUSIONS: This study demonstrates the technical feasibility of combined non-invasive electrical potential and magnetic field measurements to localize and quantitatively monitor physiological, low amplitude, infraslow cortical activity in humans. This specific combination of simultaneous recording techniques allows to benefit from the specific physical advantages of each method. SIGNIFICANCE: This combined non-invasive MEG-EEG methodology is able to provide important information on infraslow neuronal activity originating from tangentially and radially oriented sources. Moreover, this dual approach has the potential to separate neuronal from non-neuronal DC-sources, e.g., radially to the head orientated DC-currents across the skin/scalp/skull/dura occurring during cerebral hypercapnia or hypoxia.

Comparing a template approach and complex bandpass filtering for single-trial analysis of auditory evoked M100.

Two methods for single-trial analysis were compared, an established parametric template approach and a recently proposed non-parametric method based on complex bandpass filtering. The comparison was carried out by means of pseudo-real simulations based on magnetoencephalography measurements of cortical responses to auditory signals. The comparison focused on amplitude and latency estimation of the M100 response. The results show that both methods are well suited for single-trial analysis of the auditory evoked M100. While both methods performed similarly with respect to latency estimation, the non-parametric approach was observed to be more robust for amplitude estimation. The non-parametric approach can thus be recommended as an additional valuable tool for single-trial analysis.

Application of decorrelation-independent component analysis to biomagnetic multi-channel measurements.

Biomagnetic multi-channel recordings are typically a superposition of signals from several biological sources of interest and from biological and technical noise sources. Besides averaging, source localization, and spectral analysis to name only a few methods, independent component analysis is an established tool to resolve the superposition present in raw biomagnetic data on a purely statistical basis. Here the time-delayed decorrelation-independent component analysis algorithm is applied to exemplary magnetocardiographic and magnetoencephalographic data and the successful signal separation is demonstrated.

Recording of focal direct current (DC) changes in the human cerebral cortex using refined non-invasive DC-EEG methodology.

A non-invasive DC electroencephalographic (DC-EEG) method was developed to record and analyze focal low-frequency (<0.1 Hz) DC changes in the human cerebral cortex. A simple repetitive finger-movement task was used as a physiological activation paradigm. DC-EEG amplitudes were recorded using a custom-made DC amplifier with automatic offset correction. A total of 16 standard Ag/AgCl electrodes covered the left primary motor cortex. In three of six subjects, reliable focal motor-related DC-EEG shifts over the hand cortex were monitored. This study demonstrates that refined DC-EEG recording and data analysis procedures allow non-invasive recording of low-frequency and low-amplitude focal cortical changes in humans. An important clinical perspective of this technology is the detection of stroke-associated cortical DC activity.

Comparison of magnetocardiography and electrocardiography.

OBJECTIVE: Automated techniques were developed for the measurement of cardiac repolarisation using magnetocardiography. METHODS: This was achieved by collaboration with the Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany and the Grönemeyer Institute of Microtherapy, Bochum, Germany, to obtain recordings of magnetocardiograms (MCGs) in cardiac patients and healthy subjects. Manual and automated ventricular repolarisation measurements from MCGs were evaluated to determine the clinical relevance of these measurements compared with electrocardiograms (ECGs). RESULTS: Results showed that MCG and ECG T-wave shapes differed and that manual repolarisation measurement was significantly influenced by T-wave amplitude. Automatic measurements of repolarisation in both MCGs and ECGs differed between techniques. The effects of filtering on the waveforms showed that filtering in some MCG research systems could significantly influence the results, with 20 ms differences common. In addition, MCGs were better able to identify differences in the distribution of cardiac magnetic field strength during repolarisation and depolarisation between normal subjects and cardiac patients. Differences were also determined in ventricular repolarisation between MCGs and ECGs, which cannot be explained by channel/lead numbers or amplitude effects alone. CONCLUSION: The techniques developed are essential, because of the many extra MCG channels to analyse, and will encourage the use of MCG facilities.

Selection of potential iron oxide nanoparticles for breast cancer treatment based on in vitro cytotoxicity and cellular uptake.

Superparamagnetic iron oxide nanoparticles (SPIONs) are promising tools for the treatment of different diseases. Their magnetic properties enable therapies involving magnetic drug targeting (MDT), hyperthermia or imaging. Depending on the intended treatment, specific characteristics of SPIONs are required. While particles used for imaging should circulate for extended periods of time in the vascular system, SPIONs intended for MDT or hyperthermia should be accumulated in the target area to come into close proximity of, or to be incorporated into, specific tumor cells. In this study, we determined the impact of several accurately characterized SPION types varying in size, zeta potential and surface coating on various human breast cancer cell lines and endothelial cells to identify the most suitable particle for future breast cancer therapy. We analyzed cellular SPION uptake, magnetic properties, cell proliferation and toxicity using atomic emission spectroscopy, magnetic susceptometry, flow cytometry and microscopy. The results demonstrated that treatment with dextran-coated SPIONs (SPIONDex) and lauric acid-coated SPIONs (SPIONLA) with an additional protein corona formed by human serum albumin (SPIONLA-HSA) resulted in very moderate particle uptake and low cytotoxicity, whereas SPIONLA had in part much stronger effects on cellular uptake and cellular toxicity. In summary, our data show significant dose-dependent and particle type-related response differences between various breast cancer and endothelial cells, indicating the utility of these particle types for distinct medical applications.